In so-called force control in which a tip end such as an arm tip end portion of a robot (hereinafter, simply referred to as a “tip end”) controls a position and contact force thereof in a task coordinate system while contacting a subject, it is important to estimate external force that acts on the tip end, and to flexibly correct an operation of the tip end so that the operation can correspond to the external force. In the case where a force sensor is attached to the tip end, the external force can be measured highly accurately. However, since the force sensor is fragile against impact as well as is expensive, it is frequent that use of the force sensor is avoided.
In the case where the force sensor is not used, it is necessary to estimate external torque of an actuator, which is generated by an influence of the external force that acts on the robot, in order to estimate the external force. The external torque is calculated by estimating drive torque necessary for motion of the actuator and subtracting the estimated drive torque from an actual torque command value sent to a drive unit (amplifier). For example, a method is known, which calculates the drive torque from a joint command for each joint, estimates the external torque from a difference between the calculated drive torque and the torque command value, and converts the estimated external torque into the external force in the task coordinate system (refer to Patent Literature 1 (JP 405-4984)).
However, though it is necessary to highly accurately estimate parameters of a dynamics model in advance in order to estimate the drive torque, realization of such parameter estimation involves difficulty. In particular, though friction force is an item having a large influence in the drive torque, a friction coefficient thereof changes largely by an operation velocity, operation direction, operation history and operation environment of the actuator, a load attached to the tip end, and the like. Accordingly, it is difficult to estimate the friction coefficient precisely. In the motion of the actuator in the task coordinate system, an operation direction of a joint shaft thereof changes frequently and complicatedly. Accordingly, it cannot be said that external force estimation in each joint shaft, which is as described in Patent Literature 1, is just sufficient. As described above, in the case where the force sensor is not used in the external force estimation, it has been difficult to estimate the external force more highly accurately, and to provide stable force control.